Solar Physics Research at Tufts
University

The Tufts solar physics group has long been
involved in the analysis and interpretation of radio, X-ray and
optical wavelength observations of the Sun and active stars. The
main objective of this research is an improved understanding of physical
processes that occur within active regions that produce intense
solar flares and other eruptive phenomena.

We have used the world's most sensitive radio telescope, the
Very Large Array (VLA) to study the structure and evolution of
radio emission from coronal loops and solar bursts. The VLA is
unique in its ability to provide two-dimensional images of bursts
and other long-lived structures with high spatial and temporal
resolution. High resolution VLA observations of the Sun at 20 and
91 cm have made it possible to locate and resolve sites of
impulsive energy release at two different heights in the solar
corona, thereby providing insights to acceleration mechanisms and
plasma parameters of the energetic particles. Investigations of
microwave burst emission have also yielded insights to the pre-
flare heating of coronal loops, the successive interaction of
adjacent loops and the role of nonthermal particles during all
phases of flares. Our VLA data have also been compared with X-
ray observations from the Yohkoh Soft X-ray Telescope and with
Extreme Ultraviolet observations from the Solar and Heliospheric
Observatory (SOHO), the TRansition Region and Coronal Explorer (TRACE)
and the Reuven Ramaty High Energy Solar and Spectroscopic Imager (RHESSI) satellites.
These instruments sample solar
emission from different temperature regimes and at different
heights in the solar atmosphere and complement the VLA
observations and enhance its scientific return. In the picture above, we compare EUV and soft
X-ray images taken with the SOHO Extreme Ultraviolet Imaging Telescope (EIT) and the
Yohkoh SXT with a VLA snapshot map at 91 cm wavelength. The VLA map shows intense Type
I noise storm emission above the active region at the limb. This time-variable radio emission
followed, by a few minutes, a solar flare and coronal mass ejection detected by SOHO and
TRACE, This suggests that
these evolving EUV sources may have played a role in accelerating electrons that
were eventually seen in the noise storm at greater heights.

Recent Publications

Very Large Array, SOHO and RHESSI Observations of Magnetic Interactions and
Particle Propagation ACross Large-SCale Coronal Loops

Very Large Array Observations of Evolving Type I Noise Storms and Nonthermal Bursts In Association
With Flares and Coronal Mass Ejections

Very Large Array (VLA) observations of the Sun at 20, 91 and 400 cm have been
combined with data from the SOHO, TRACE and Wind solar missions to study the properties of
long-lasting Type I noise storms and impulsive metric and decimetric bursts during solar flares
and associated coronal mass ejections. These radio observations provide information about the
acceleration and propagation of energetic electrons in the low and middle corona as well as their
interactions with large-scale magnetic structures where energy release and transport takes place.
For one flare and its associated CME, the VLA detected impulsive 20 and 91 cm bursts that were
followed about ten minutes later by 400 cm burst emission that appeared to move outward into
the corona. This event was also detected by the WAVES experiment on Wind which showed
intense, fast-drifting interplanetary Type III bursts following the metric and decimetric bursts
detected by the VLA. For another event, impulsive 91 cm emission was detected about a few
minutes prior to impulsive bursts at 20.7 cm, suggesting an inwardly propagating beam of
electrons that excited burst emission at lower levels and shorter wavelengths We also find
evidence for significant changes in the intensity of Type I noise storms in the same or nearby
active region during impulsive decimetric bursts and CMEs. These changes might be attributed to
flare-initiated heating of the Type I radio source plasma by outwardly-propagating flare ejecta or
or
to the disruption of ambient magnetic fields by the passage of a CME.

Very Large Array (VLA) observations of the Sun at 91 and 400 cm wavelength have been used to
investigate the radio signatures of EUV heating events and coronal mass ejections (CMEs)
detected by SOHO and TRACE. Our 91 cm observations show the onset of Type I noise storm
emission about an hour after an EUV ejection event was detected by EIT and TRACE. The EUV
event also coincided with the estimated start time of a CME detected by the LASCO C2
coronagraph, suggesting an association between the production of nonthermal particles and
evolving plasma-magnetic field structures at different heights in the corona. On another day, our
VLA 400 cm observations reveal weak, impulsive "microbursts" that occurred sporadically
throughout the middle corona. These low brightness temperature (Tb = 0.7 -2.2 x 106 K) events
may be weak Type III bursts produced by beams of nonthermal electrons which excite plasma
emission at a height where the local plasma frequency or its first harmonic equals the observing
frequency of 74 MHz. For one microburst, the emission was contained in two sources separated
by 0.7Ro, indicating that the electron beams had access to widely-divergent magnetic field lines
originating at a common site of particle acceleration. Another 400 cm microburst occurred in an
arc-like source lying at the edge of EUV loops that appeared to open outward into the corona,
possibly signaling the start of a CME. In most instances the 400 cm microbursts were not
accompanied by detectable EUV activity, suggesting that particles that produce the microbursts
were independently accelerated in the middle corona, perhaps as the result of some quasi-
continuous, large-scale process of energy release.

We discuss the results of recent collaborative investigations of the Sun that ombine Very Large
Array (VLA) images at 91.6 cm wavelength with SOHO EIT and TRACE EUV
spectroheliograms at several wavelengths. The main objective of these observations was to
investigate the spatial and temporal relationship between nonthermal
decimetric burst activity and small-scale transient EUV events in active regions. Our 91 cm
observations show changes in the intensity of overlying Type I noise storms a few minutes after
five of seven transient brightenings observed by SOHO and TRACE. This suggests that these
evolving EUV sources may have played a role in the production of nonthermal electrons
eventually seen in the noise storm emission at greater heights.

The First VLA Observations of Nonthermal Metric Bursts
Associated With Coronal Mass Ejections Detected By SOHO

We present the first observations of nonthermal decimetric
burst emission of the Sun
using the new 400 cm (74 MHz) system at the VLA. Our VLA
observations were carried out in collaboration with the Large
Angle Spectroscopic Coronagraph (LASCO) and the
Extreme-ultraviolet Imaging Telescope (EIT) on board the SOlar
and
Heliospheric Observatory (SOHO) spacecraft. Full-disk
observations at 400 and 91 cm were used to study the spatial
and temporal variations of nonthermal radio bursts during
Coronal Mass Ejections (CMEs) detected by LASCO as well as
transient Extreme Ultraviolet (EUV) brightenings
detected by the EIT. VLA snapshot maps at 400 cm revealed
impulsive burst emission in the low corona that began near the
estimated start time of the CME activity; that beginning also
coincided with a C1.1 GOES X-ray burst and an EIT flare-like
brightening. The nonthermal
metric bursts then continued sporadically during the next several
hours, which included the ejection of spatially separated CME
components. The 400 cm bursts
are contained within curved, or arch-like, sources at a fixed
radial distance, but with a varying position between two
active regions detected by the EIT near the limb, suggesting that
they were emitted within large-scale magnetic loops. Our 91
cm observations also show the onset of a long-lasting Type I noise
storm
following the initiation of CME activity, again suggesting an
intimate
relationship between the production of nonthermal particles
and large-scale evolving plasma-magnetic field structures in the
corona.

The Radio and EUV Signatures of Small-Scale Coronal
Magnetic Reconnection Events

We discuss the results of recent collaborative investigations of
evolving solar active regions that combine Very Large Array
(VLA)images at 3.5,
6.2, 20.7 and 91.6 cm with SOHO EIT and CDS EUV spectroheliograms
at several wavelengths. The main objective of these observations
was to study the spatial and temporal relationship of transient
events in both the radio and EUV domains and to constrain
physical parameters such as electron temperature, electron
density and magnetic field strength in emitting regions.
Observations made on eight different days in 1997 show that only
some EUV events have counterparts in microwaves. The EUV
transient events seem to occur throughout the target active
regions while the evolving microwave sources predominantly
overlie regions of high magnetic field strength near sunspots.
Our VLA 91 cm observations also show changes in the brightness
of Type I noise storm and decimetric Type III-like burst sources
following some
EUV transient events. In one case, significant changes in the
structure of
20 cm coronal loops and 91 cm noise storm sources occurred a few
tens of minutes prior to the appearance of an EUV ejection event
or jet suggesting interaction
between large and small-scale loops at different levels in the
corona.

RATAN-600 spectral-polarization observations
of solar active regions have been compared with with VLA and
Nancay Radioheliograph observations of noise storms and Yohkoh
soft X-ray images of coronal loops. The RATAN data were
used to identify active regions which show narrow-band
polarization inversions of
sunspot-associated microwave sources. Several different types of
spectral and temporal changes of the microwave polarization of
the local sources associated with noise storm-producing active
regions were detected.In contrast to some previous observations,
our data show that the time at which the polarization inversion
occurs does not always depend on the location of the active
region on the disc with respect to central meridian passage. For
one active region the inverted polarity of a sunspot-associated
source persists for several days and then reverts to what one
would normally expect from the polarity of the underlying
sunspot. For other microwave sources, a double inversion in
circular polarization was observed over a relatively narrow
frequency range. The
observed inversion of the sign of circular polarization was, in
some cases, impossible to explain in terms of the propagation of
radio waves through a quasi-transverse (QT) region in the corona.
Instead, these narrow-band polarization changes require more
complicated magnetic configurations, such as current sheets,
that may also produce the accelerated particles responsible for
noise storms at longer wavelengths

VLA-SOHO Observations of Evolving Coronal Structures on
the Sun

We discuss preliminary results of collaborative VLA and SOHO
observations of the Sun made during two Joint Observing Programs
in 1996 and 1997. VLA images at 3.5, 6.2, 20.7 and 91.6 cm were
combined with SOHO EIT, CDS and SUMER spectroheliograms to
delineate the structure of magnetic loops in the transition
region and corona and to constrain the emission mechanism and
plasma parameters in these
regions. VLA snapshot maps at 3.5 and 6.2 cm, together with
sequences of EIT and CDS images (2' - 8.5' fd of view), were
also used to study temporal variations in source structure that
reflect changes in temperature and magnetic
field strength of the transition region and corona. Full-disk VLA
maps at 20 cm wavelength reveal compact (10" - 20") sources that
may be the radio counterparts of EUV bright points. The 91 cm VLA
maps show extended emission from the quiet Sun including
brightness temperature depressions above coronal holes seen by
EIT at EUV wavelengths. These radio maps also show large-scale
(5' - 7') loop-like structures at the solar limbs that lie at the
base of evolving coronal streamers detected by the LASCO
coronagraphs

X-ray Jets and Their Radio Signatures at Metric and
Centimeter Wavelenths

On October 19, 1995, two homologous X-ray jets were observed with
the
Yohkoh/SXT from AR 7912, a region having a reversed polarity
group with high magnetic shear. The high shear is well-visible
in the vortex-like H$\alpha$ fibril pattern observed with the
MSDP instrument mounted on the German VTT
on Tenerife as well as in twisted-looking X-ray loops on the
Yohkoh images.
The first event, between 10:25-10:32 UT, was
observed with the Nancay radio heliograph at 167, 236 and 327
MHz. Type III activity superimposed on a noise-storm was clearly
visible at 164 MHz and 327 MHz. At 164 MHz and 236 MHz
the storm had a double structure, while at 327 MHz it consisted
of a single component. Type III activity first appeared at 164
MHz and 236 MHz close to the storm position and in the direction
of the X-ray jet. Between 10:28:40 UT and 10:28:50 UT a new group
of sources appeared eastward of the former activity, which may
correspond to remote footpoints of a large-scale loop linked to
the active region or to a second jet event along a more eastern
path. At 327 MHz burst activity was observed near the position
of the storm, and close to the location of the first type III
burst,along the X-ray jet.
The second X-ray jet event occurred between 16:58-17:12 UT, and
was also observed with the Very Large Array (VLA) at 6.2, 20.7
and 91.6 cm. For this event, VLA snapshot maps at 6.2 and 20.7 cm
reveal low-brightness temperature changes in source structure at
the site of the X-ray jet during the preburst, impulsive and
decay phases. The VLA 91.6 cm observations also
show noise storm emission above the active region but
there is no clear temporal correlation between this later X-ray
jet and the impulsive decimetric bursts that were observed during
this period. Although the X-ray observations show that the two
jets had similiar temperatures and emission measures, they
appear to have had dissimiliar centimeter and metric responses to
these events and the implications of these results will be
discussed in our paper.

VLA-SOHO OBSERVATIONS OF THE SOLAR
CORONA

K.R. Lang, R.F. Willson

(paper presented at the 5th SOHO Workshop, Oslo, Norway)

Abstract

Very Large Array (VLA) observations at 3.5, 6.2, 20.7 and 91.6
cm wavelength are compared
with SOHO SUMER, CDS, EIT and LASCO data taken during three days
of
coordinated observations in June, 1996. VLA synthesis maps at
these
wavelengths have been combined with SUMER and CDS
spectroheliograms to delineate
the structure of a target active region from the chromosphere to
corona and
to constrain the emission mechanism and plasma parameters at
these levels. Snapshot maps at 3.5 and 6.2 cm
have also been used to study temporal variations in the emission
that may reflect changes
in the temperature or magnetic structure in the underlying active
region.
Full-disk VLA maps at 20 cm wavelength reveal a number of compact
(10"-20") low brightness sources that may be the radio
counterparts of EUV
bright points detected by the EIT. The 91.6 cm VLA maps show
extended emission across the solar disk including brightness
temperature depressions
that lie above coronal holes seen in the EIT images. These maps
also show loop-like
structures at the solar limbs that appear to lie at the base of
evolving coronal streamers detected by the LASCO
coronagraphs.